What does it really cost to trade a stock? The price you see on a screen is only part of the story. Behind every trade are hidden costs — the effort of finding someone to take the other side, the way your order moves the price, and the fact that some stocks trade far less often than others. This chapter shows how we measure those costs and what they mean for the returns investors earn. It turns out that stocks that are harder to trade tend to pay investors more over time. So liquidity — how easy it is to trade — is not just a convenience; it is a risk factor that investors care about and that affects stock returns.